1. Field of the Invention
The present invention relates to a MOSFET having a drain voltage detection function.
2. Description of the Related Art
A protection circuit is conventionally incorporated in an integrated circuit to detect a current flowing through a MOSFET located at an output portion of the integrated circuit and to protect a main element against an overcurrent. In a known conventional method, a part of a source electrode is divided to be used as a current detection terminal, thereby detecting a current flowing through the MOSFET at the output portion More specifically, if the MOSFET is of an n-channel type, a p-type base layer is divided into a plurality of islands, and an n-type source layer is formed on each p-type base layer. A plurality of source and base layers are commonly connected to a source electrode. In such a structure, a specific source electrode portion of one p-type base layer is separated from the other p-type base layers so as to be used as an overcurrent detection electrode. With this arrangement, since the source electrode is not short-circuited to the detection electrode, the potential of the detection electrode increases independently of the potential of the source electrode with an increase in drain voltage. Therefore, by inserting, e.g., a resistor between the detection electrode and the ground potential and detecting a voltage drop across this resistor, a current flowing through the element can be detected.
In the case of, especially, a conductivitymodulation MOSFET, if a p-type base layer is divided into a plurality of islands, a latch-up withstand voltage is decreased. As a result, a current value capable of turning off the conductivity-modulation MOSFET is decreased. Therefore, it is preferred to continuously form a p-type base layer and continuously form a source layer in the p-type base layer. In such a structure, however, the above method of simply dividing a source electrode to form a detecting electrode cannot be used because the potential of the detection electrode is not independent from that of the source electrode.
As described above, the overcurrent detection method of dividing a source electrode to form a current detection electrode as used in the conventional output portion MOSFET is problematic because it cannot be applied to an element having a continuous p-type base layer.